For traditional forage use, native warm-season grasses are highly productive in the summer months and are suited for a complementary role with cool-season forages in full-season forage systems of the Midwest. For renewable energy sources, these grasses can be used to produce biofuels, an alternative energy source in the future for fossil fuel which is becoming less available. Farmers may integrate forage and biomass cropping for flexibility and diversity in their farming systems. For dual-purpose crops (forage or biomass), the production of warm-season grasses would provide farmers some protection against the market failure of a single purpose. The goals for the production of warm-season grasses as biomass differ from those when used as forage. The goal for biomass production is to maximize the concentration of lignocellulose in the feedstock and minimize the concentration of N and minerals. To achieve the production potential, optimal management practices are required.

The purposes of this research were to determine in some detail the yield and quality of four native warm-season grasses as influenced by nitrogen application and harvest timing for (i) forage and (ii) biomass production.

In the first study (Chapter 3), we investigated the nutritive quality of native warm-season grasses as influenced by N fertilization rate and harvest timing. Forage quality of native warm-season grasses decreases as the plants mature. For all species, harvesting at early maturity increased forage quality including increased IVDMD and crude protein, and decreased NDF concentrations, whereas allowing forage to reach later maturity reduced forage quality with declines in IVDMD and crude protein and increases in NDF concentration. There were variations in NDF and IVDMD concentration among warm-season grass species in response to N application rates, but none was observed for crude protein. The concentration of crude protein for four warm-season grass species increased as higher rates of N fertilization. Therefore, although there were variations in the effect of N fertilization on NDF and IVDMD making drawing conclusions difficult, N fertilization could improve forage quality for warm-season grasses through increasing the concentration of crude protein in warm-season grasses. Across big bluestem, switchgrass, and indiangrass, quadratic regression equations used for predicting the changes in forage quality parameters from mean stage count (MSC) had high coefficients of determination yy 0.96 for IVDMD, 0.88 for NDF, and 0.88 for CP, respectively. However, predicting forage quality changes based on MSC did not provide reasonable estimates for eastern gamagrass.

In the second study (Chapter 4), we determined optimum N fertilization rates and harvest timing for the biomass production of native warm-season grasses and investigated if traits of canopy architecture including mean stage by count and tiller demographics of native warm-season grasses can explain observed yields of warm-season grasses with varied harvest dates and N fertilization rates. Biomass yields increased with advancing maturity but differently among species before declining during senescence. Eastern gamagrass obtained the maximum yield at the highest mean stage count (MSC; 1.6 and 2.2) when the largest seed ripening tillers are present. Big bluestem, switchgrass, and indiangrass are more determinate in their growth patterns. Their maximum yields occurred at MSC 3.5, 3.9, and 2.9, respectively when the largest reproductive tillers are present. Delaying harvest to late fall may improve biomass quality, but it decreased yield due to a large proportion of senescenced tillers and increased litter. In terms of a biomass supply strategy, eastern gamagrass may be used as a feedstock in early summer, while big bluestem and switchgrass may be used between mid-and late summer, and indiangrass in early fall. Nitrogen fertilization increased yield by increasing the proportion of elongating, reproductive, seed tillers for determinate grasses (big bluestem, indiangrass, and switchgrass) and increasing tiller density for indeterminate grass (eastern gamagrass).

In the third study (chapter 5), we determined biomass quality of four warm-season grasses as influenced by nitrogen fertilization rates and harvest timing. In both years, delaying harvest dates increased cellulose, lignin, and C concentrations, but decreased ash and N concentrations, however, the magnitude of these effects varied among species. The concentration of N, C, cellulose, and lignin increased, whereas ash content declined with increasing rates of N fertilization, but also varied among species. Our results indicated that the warm-season grasses supplied with N fertilizer at 140 kg ha-1 and delayed to harvest until fall provide the optimal composition for biomass feedstock production.

Therefore, the four native warm-season grasses in this study could serve dual purposes as forage when harvested at early maturity, and as biomass feedstocks when harvested during late maturity. Optimal N fertilization could provide economically feasible alternative uses of these warm-season grasses by improving both total yields and quality of forage and biomass.